As materials of devices, which are used for boilers, chemical plants and the like, under a high temperature environment, 18-8 austenitic stainless steels such as SUS304H, SUS316H, SUS321H and SUS347H, have been used. In recent years the use conditions of these devices under such a high temperature environment, have become remarkably severe. Accordingly the required properties for the materials used in such an environment have attained a higher level. The conventional 18-8 austenitic stainless steels are insufficient in high temperature strength, particularly in creep strength, so in these circumstances, an austenitic stainless steel, having improved high temperature strength by adding the particular amounts of various elements, has been proposed.
For example, an austenitic stainless steel in which high temperature strength was significantly improved by adding the comparatively inexpensive Cu together with Nb and N in proper amounts, has been proposed in Publication of examined Patent Application No. Hei 8-30247, Publication of unexamined Patent, Application No. Hei 7-138708 and Publication of unexamined Patent Application No. Hei 8-13102. In this steel Cu precipitates coherently with the austenite matrix during use at high temperatures, and Nb precipitates as complex nitiride with Cr, NbCrN. Since these precipitates very effectively act as barriers against the dislocation movement, the high temperature strength of the austenitic stainless steel is enhanced.
However, in the field of the thermal power generation boiler, a project which increases the vapor temperature to between 650° C. and 700° C., wherein the temperature of the material for parts far exceeds 700° C., has been recently promoted. Therefore, the austenitic stainless steels proposed in the above-mentioned Patent Documents will be insufficient in various properties. In other words the above-mentioned Cu, Nb and N added steels, as materials for being able to endure in the said environment of high temperature and high pressure, are still insufficient in high temperature strength and corrosion resistance. Particularly, there is also another problem, which is the toughness of the steel, after being used at high temperatures of 800° C. or higher for long period, is insufficient. Further, the hot workability of the Cu, Nb and N added steels is inferior to that of the conventional 18-8 austenitic stainless steel, therefore an prompt improvement of the steels is required.
Some steels, in which hot workability has been improved to some extent, have been proposed. For example, in Publication of unexamined Patent Application No. Hei 9-195005, a steel in which the hot workability is enhanced by adding one or more of Mg, Y, La, Ce and Nd, has been proposed. In Publication of unexamined Patent Application No. 2000-73145 and Publication of unexamined Patent Application No. 2000-328198 steels in which the hot workability is enhanced by adding proper amounts of Mn, Mg, Ca, Y, La, Ce or Nd, in accordance with the amounts of Cu and S, have been proposed. Further, in Publication of unexamined Patent Application No. 2001-49400, a steel in which the tube making properties, in a hot rolling method such as the Mannesmann mandrel mill process, are improved by adding B (Boron), under limitation of S to 0.001% or less, and O (Oxygen) to 0.005% or less, and further adding Mg or Ca in proper amounts, in accordance with the amounts of S and O has been proposed.
However, these steels are insufficient in the improvement of hot workability. Particularly, the workability at temperatures of 1200° C. or higher has not been improved.
Generally, a material having poor hot workability is formed into a seamless tube by hot extrusion. Since the internal temperature of the material becomes higher than the heating temperature, due to the heat produced by working, material having insufficient workability at 1200° C. or higher generates cracks, so-called lamination, and inner defects. This phenomenon is the same as in a piercing by the piercer in the Mannesmann mandrel mill process and the like.